1. Field of the Invention
The invention relates to thermal insulation with a double-walled housing with an evacuated space between the two housing walls which space contains at least one fiber-like insulating material which retards the heat flow between the two housing walls and is arranged perpendicularly to the temperature gradient.
2. Description of the Prior Art
Thermal insulation with a double-walled housing finds application, among other things, especially in devices in which heat losses are to be avoided. Thermal insulation is used, for instance, in high-temperature storage batteries of the alkali metal and chalcogen type. The storage cells which are connected together to form a battery, are surrounded by thermal insulation to maintain the required operating temperature of the storage cells which is approximately 400.degree. C. and to prevent the storage cells from being cooled down especially in the operating pauses of the high-temperature storage battery, so that also after the pauses a temperature of 350.degree. to 400.degree. C. prevails in the storage cells, which is a necessary prerequisite for optimum operation of the storage cells.
German Patent Application P 32 35 708.7 (U.S. application Ser. No. 536,178, filed Sept. 27, 1983), discloses thermal insulation which is bounded by a double-walled housing. Between the two housing walls, an evacuated space is provided, into which an insulating compound is filled which retards the heat flow between the housing walls and is arranged perpendicularly to the temperature gradient. In the arrangement described here, the insulating material consists of fibers, preferably of glass fibers. The heat transport in such thermal insulation, the insulating material of which consists of fibers, is composed, with sufficiently low residual gas pressure within the evacuated space, of a solid-body conduction component and a radiation component. The thermal resistance determining the solid-body conduction is formed by the contact surfaces of the glass fibers aligned perpendicularly to the temperature gradient. If the insulating material is subjected to high temperatures, i.e. a temperature of more than 350.degree. C. prevails in the interior of the thermal insulation, the solid-body conductivity increases with rising temperature in case of additional pressure loading of the insulating material, as can be determined by means of measurements. This is found particularly in insulating material which is made of glass fibers manufactured from boron silicate glass. This increase of the solid-body conductivity is due to an increase of the fiber contact areas, since the modulus of elasticity of fibers made of commercially available glasses drops exponentially with increasing temperature.
By using fibers which are made of fused silica or ceramic, this disadvantage can be avoided, but thermal insulation, the insulating material of which is made of these materials, is very expensive.